The present invention relates generally to tactile sensors. More particularly, various embodiments of the invention describe novel tactile sensor arrays configured for elasticity imaging of soft tissue by recording a stress pattern on the tissue surface when a compression load is applied thereto.
Tactile imaging is a recently developed medical imaging technique used to improve upon and reduce the subjectivity in clinical palpation. It is used for external or internal evaluation of various soft tissues of the human body. In particular, tactile imaging is advantageous in objective evaluation of breast tissue for the presence of lumps. When a woman visits her physician, part of the physical exam often includes a clinician attempting to palpate the patient for any lumps or changes in the breast tissue that could indicate the presence of a tumor. This manual palpation method, however, only gives the physician a vague sense of what is actually underneath the skin. Due to the lack of any precise measuring device, if a lump is found through palpation, typically all that can be documented is its general location on the breast and a rough estimate of size.
To solve this problem, various tactile imaging systems have been proposed in the prior art by the inventors of the present invention as well as by others. A typical tactile imaging system consists of a hand-held device, referred to as a tactile probe, which replaces the physician's fingertips with an array of tactile sensors. When this tactile probe is used to compress the tissue, the contact pressure between the patient and the tactile probe is recorded by a computer. Simultaneously, an optional position tracker may be used to record the location of the probe such that the spatial distribution of the stress patterns can be recorded. Presence of lumps in the tissue can then be identified with high precision allowing a more informed clinical diagnosis to be made.
A typical tactile array of the tactile probe may be based on capacitance-measuring individual tactile sensors. Such sensors may be formed by providing a first electrode layer with rows of electrodes and a second electrode layer with columns of electrodes and a compressible dielectric layer therebetween. Individual sensors are formed in locations where rows of electrodes of the first electrode layer cross over columns of electrodes of the second layer. Monitoring capacitance of the sensors (such as for example by analyzing voltage between the electrode layers at each location of the sensor) allows recording of a stress pattern when the sensor is pressed against the tissue. Such tissue compression causes uneven compression of the dielectric layer which is translated into a number of capacitance measurements for respective sensors of the tactile array.
Using such technology for evaluation of the elasticity of the human tissue presents a number of unique challenges. One important requirement for a tactile probe is high consistency and reproducibility of results. The tactile sensor array of the tactile probe has to be designed to provide stable results despite variations of temperature or the method of probe handling which may somewhat change from one clinician to the next. Another important requirement is to provide a tactile probe with high sensitivity so as to detect even a faint difference in the tissue stress pattern which may be indicative of a small or deeply located tumor.
As can be appreciated by those skilled in the art, it is difficult to provide a tactile probe which is both highly sensitive and at the same time demonstrates repeatable and stable results as increasing sensitivity tends to cause an increase in artifacts and drifts caused by changing temperature or other environmental factors.
Ferroelectrets also known as piezoelectrets, are recently-developed thin films of polymer foams, exhibiting piezoelectric and pyroelectric properties after electric charging. Ferroelectret foams usually consist of a cellular polymer structure filled with air. Polymer-air composites are elastically soft due to their high air content as well as due to the size and shape of the polymer walls. Their elastically-soft composite structure is an essential key for the working principle of ferroelectrets, besides the permanent trapping of electric charges inside the polymer voids. The elastic properties allow deformations of the electrically charged voids. However, the composite structure can also possibly limit the stability and consequently the range of applications.
The most common effect related to ferroelectrets is the direct and inverse longitudinal piezoelectricity. In these cellular polymers, stress applied normally to the surface of the polymer film generally decreases the thickness of the sample. The thickness decrease occurs across the voids of the polymer causing to decrease the electrode charges. This effect makes ferroelectrets useful as a tactile sensor as they provide high sensitivity to applied pressure. At the same time, air presence makes ferroelectrets hard to work with as thermal expansion of microscopic air pockets causes the drift of the sensor reading.